Spiking neuron circuits are inspired by the functioning of neurons in the nervous systems of animals. A simple and effective way of modeling such neurons is as integrate-and-fire units: The neuron integrates input current on a capacitor until the voltage on the capacitor reaches a predetermined threshold voltage. Then, the neuron generates a spike signal and resets the voltage on the capacitor to a reference potential (typically zero voltage, or ground potential). See generally, C. Mead, Analog VLSI and Neural Systems, Addison-Wesley, Reading Mass., 1989, especially chapters 4 and 12.
Commercial importance of neuron circuits arises, in part, because they can be used as building blocks in neuronal networks for pattern recognition, as voltage-controlled or current-controlled oscillators, or as monostable pulse-generating circuits. In practice, neuron circuits are often used in arrays, as in U.S. Pat. No. 4,961,002 issued Oct. 2, 1990 to S. M. Tam, et al. It is therefore a useful attribute of neuron circuits that they be fabricated using a small number of devices, and that such devices dissipate a minimum of heat while operating. Yet, many prior implementations of neuron circuits are very complex and consume energy at such levels as to preclude packaging in densities suitable to many tasks.
In operating in an analogous way to biological neurons, neuron circuits often receive input signals from one or more sources, analogous to dendrites in biological contexts. The input signals are typically low-level signals that are often weighted to form logical functions for use in neural networks. A characteristic of most integrate-and-fire circuits is a reset of accumulated input signals upon firing at the output. Prior art neuron circuits typically perform this resetting by involving inputs in a positive feedback loop used in generating the neuron circuit spiking signal. However, such feedback can cause undesirable effects on input signal sources during neuron circuit firing and during a transient period following firing. Such feedback, and the effects of input signals arriving during the output pulse duration, can undesirably alter the charging operations of input capacitors and can adversely affect input signal sources.
Prior art neuron circuits have exhibited limitations respecting the pulse width, shape for neuron circuit outputs and the threshold voltage for neuron circuits. Additionally, it has not proven possible in some neuron circuits to control the refractory period, i.e., the time elapsed between the termination of an output pulse and the generation of a new output pulse.